Conventionally, a magneto-optical recording medium has been put into practical use as a re-writable optical recording medium. In such a magneto-optical recording medium, recording and reproducing operations are carried out by using a converged light beam released from a semiconductor laser. However, one of the disadvantages with the magneto-optical recording medium of this type is that when the diameter of a recording magnetic domain or the interval of the recording magnetic domain is smaller than the diameter of the light beam, the reproducing characteristic deteriorates.
This is caused by the fact that since a light beam, which is converged on a target recording magnetic domain, includes other recording magnetic domains adjacent to the recording magnetic domain in its beam diameter, individual recording magnetic domains are not reproduced in a separated manner.
Japanese Laid-Open Patent Publication No. 150418/1994 (Tokukaihei No. 6-150418) has proposed an arrangement for overcoming the above-mentioned disadvantage with the magneto-optical recording medium. In this arrangement, a reproducing layer, a non-magnetic intermediate layer and a recording layer are stacked in this order. The reproducing layer exhibits in-plane magnetization at room temperature and comes to exhibit perpendicular magnetization with temperature rises. Further, in this arrangement, the reproducing layer and the recording layer are magnetostatically coupled with the non-magnetic intermediate layer located in between, and a portion of the reproducing layer, which is in a perpendicular magnetization state, copies the magnetization of the recording magnetic layer. The other portion thereof, which is in an in-plane magnetization state, masks the magnetization of the recording layer. For this reason, in the recording layer, information in recording magnetic domains that are adjacent to a portion of the reproducing layer that is in the in-plane magnetization state is not reproduced. Therefore, even if recording magnetic domains that are to be reproduced and that are adjacent to the recording magnetic domains are included in the diameter of a light beam, individual recording magnetic domains are reproduced in a separated manner; that is, a magnetic super-resolution reproducing operation is available.
In the magneto-optical recording medium disclosed in the above-mentioned patent publication, it is necessary to use a rare-earth transition-metal alloy with a RErich (Rare Earth Metal Rich) composition at room temperature as a material for the reproducing layer. This alloy allows the reproducing layer to obtain the magnetic characteristic that exhibits in-plane magnetization at room temperature while exhibiting perpendicular magnetization with a temperature rise. With respect to the material for the recording layer, it is preferable to use a rare-earth transition-metal alloy with a TMrich (Transition Metal rich) composition in the vicinity of the Curie temperature.
Here, the RErich composition refers to a composition wherein as compared with the compensation composition of a rare-earth transition-metal alloy, the magnitude of the sublattice moment of the rare-earth metal (RE) is greater than that of the sublattice moment of the transition metal (TM). The compensation composition is a composition wherein the magnitude of the RE sublattice moment balances the magnitude of the TM sublattice moment in a rare-earth transition-metal alloy composition. In the same manner, the TMrich composition refers to a composition wherein the transition metal has a greater sublattice moment.
The resulting problem with the above-mentioned magneto-optical recording medium is that the reproducing polarity tends to be inverted, as compared with a magneto-optical recording medium without a reproducing layer. This is because the reproducing layer with the RErich composition and the recording layer with the TMrich composition are magnetostatically coupled so as to make the directions of the total magnetizations of both of the layers coincide with each other, and the directions of the TM sublattice moments of the reproducing layer and the recording layer are nonparallel to each other. For this reason, in order to allow the magneto-optical recording medium of the above-mentioned patent publication to be compatible with a magneto-optical recording medium without a reproducing layer, it is necessary to take the polarity inversion into consideration in providing recording and reproducing operations.